To be honest, I think that Brawl and Melee pretty much brought in any All-Star characters from Veteran franchises that are really necessary, with a few exceptions.
Mario: Yes, it is Nintendo's flagship IP. But the gang's all there. If anything, Toad should be promoted to Assist Trophy, in my opinion, but he's also fine where he is. As much as I'd like Waluigi in, I'd MUCH rather see new franchises altogether. And Bowser Jr...meh. Don't get me wrong, Bowser Jr. is probably my favorite Mario character, but I just don't see it happening. Same with Paper Mario. Mario, Luigi, Peach, Bowser, and all the spinoff characters rep the series fine.
No matter how famous Mario is, we have to make room for the little guys. Let's try to remember that.
No Geno.
DK: This series definitely deserves three reps. I'm really torn between K. Rool and Dixie, but I personally like King K. Rool more.
Yoshi: He's fine where he is. There isn't anyone from the Yoshi series so deserving that Sakurai should neglect a more obscure, yet more deserving, character.
Wario: Same as Yoshi. Though, a lot of the WarioWare cast would make for terrific Assist Trophies.
Legend of Zelda: Fine where it is, even with Toon Link. If Toon Link goes, the only other understandable choice would be Tingle, due to his popularity (and/or infamy) and the fact that he appears in quite a few games. The problem with Vaati is that he is Toon Link's antagonist. You don't replace Toon Link with his own foe. It's weird. Ghirahim, no matter how fabulous, is a one-time creep. Same with Skull Kid, although he is comparably iconic. COMPARABLY.
Five is good enough. Keep Brawl's Zelda roster, but make Ganondorf more unique. IF they take out Toon Link, then add Tingle. You decide which is the lesser of two evils.
Pokemon: Since this is such a HUGE and fluid franchise, five character slots, I could understand. If you were to keep Brawl's Pokemon roster, and add Mewtwo, that would be perfect.
Kirby: Fine where it is.
Metroid: Having been around since the original game, I could understand seeing Ridley. He is iconic in his own right. But no one else. Samus and Ridley (and ZSS).
Star Fox: The three we have is fine. But if we made them differ even more than Brawl did this time around, that'd be fantastic.
Kid Icarus: The only worthy second rep would be Palutena, BUT I think it'd be better if she were a codec-esque character. Witty banter with Pit about his adversaries would be terrific.
Fire Emblem: Fire Emblem should really have two reps. If they can give an Awakening character a unique moveset, then leave in Ike (I doubt Marth's going anywhere) and give us three, but other than that, just two.
Mother: Before I go on, I'm a huge fan of this franchise, so I know a thing or two about the level of importance, when it comes to the characters. Hypothetically, on the off chance that Mother perhaps gets a third rep MAYBE, it should be Porky. He can be just as unique as Masked Man, except he's actually in more than one game. He's a staple of the series.
Now, notice how I said "Hypothetically", "off chance", "perhaps", and "MAYBE". That's because Mother doesn't really need one. A cult-classic franchise with a grand total of three games is fine where it is. I should be grateful it got a boss in Brawl! (and it was Porky, of all people!) I know Mother's the bee's knees in Japan, but it still applies.
Pikmin: There is no other character in the Pikmin franchise that has the potential to add anything unique to Smash Bros. Every playable character is literally just a cartoony unfortunate space person who pulls carrot people out of the ground and makes said carrot people rebuild spaceships. Olimar, having been in every game, is the one and only ideal rep.
F-Zero: As much as I love the series, there isn't a character unique and iconic enough to fill the role of a playable character, nor is their any incentive to add a character from a (all but) dead franchise. I wish it wasn't dead...
I think I pretty much hit that nail on the head. In terms of vet-franchise newcomers, Ridley, K. Rool, and Mewtwo (maybe Chrom or something, but I'm pretty content with Marth and Ike) should be it.